Optical recording discs in the prior art include non-erasable write-once systems which utilize as the active recording layer a TeO.sub.x (0&lt;.times.&lt;2.0) thin film formed from Te and TeO.sub.2. Erasable discs have also been reported and are being developed for practical applications in which it is possible to repeatedly write and erase information by optical means. In such erasable optical discs, a thin film layer of material is typically heated and melted by laser light, then rapidly cooled so that its structure is in a substantially non-crystalline or amorphous state, thereby recording information which is indicated by the optical properties of the substantially non-crystalline or amorphous state. The recorded information can be subsequently erased by heating the active layer, and then slowly cooling it so that its atomic structure anneals and transforms into a substantially crystalline state, having different optical properties from that of the amorphous state, which indicate thereby an erased condition.
Materials investigated as active layers for erasable discs which operate via a phase change mechanism involving an amorphous/crystalline transition include various combinations of the chalcogen elements as exemplified by Ge.sub.15 Te.sub.81 Sb.sub.2 S.sub.2. Such combinations have been studied by Ovshinsky et. al and Feinleib et al. (see Appl. Phys. Lett., vol. 18 (1971)). In addition, thin film active layers consisting of combinations of a chalcogen element or elements with an element or elements chosen from Group V of the periodic table or an element or elements chosen from Group IV of the periodic table, e.g. Ge, As.sub.2 S.sub.3, As.sub.2 Se.sub.3 or Sb.sub.2 Se.sub.3 are known and have been studied in the prior art.
It is possible to produce an optical disc having thin film active layers on a substrate in which grooves are formed for the purpose of guiding the laser light. With respect to the utilization of such optical disc for the recording and erasing of information by laser light, the active layer is generally crystallized in advance, and a laser beam focused to a spot size of about 1 micron is intensity modulated between a peak power level and a lower bias power level with the recorded information. For example, a circular recording disk may be rotated and irradiated during rotation with pulses of laser light having a peak power sufficient to increase the temperature of the irradiated areas on the active layer above the melting point of the layer. If the irradiated areas are permitted to cool rapidly, the information will be recorded by the formation of substantially non-crystalline or amorphous marks at the locations of the irradiated areas.
Amorphous areas of the disc which are irradiated with the lower bias power level of the laser light can have the temperature in those areas elevated above the crystallization temperature of the active layer, in which case the active layer at those irradiated areas will be transformed back into a substantially crystalline structure, and the recorded information will thereby be erased, making it possible to over-write information. In this manner, areas on the active layer may be repeatedly cycled above the melting point thereof to produce recorded amorphous areas, or above the crystallization temperature thereof to produce crystalline erased areas, thereby effectuating the recording or overwriting of binary information.
Typically, the active layer in an optical disc is sandwiched between dielectric layers which have excellent heat resistance characteristics. These dielectric layers serve to contain the active layer and to protect a substrate and an adhesive layer from undergoing large changes in temperature during irradiation. Since the thermal behavior of the active layer, both as to it its ability to rapidly increase in temperature, as well as its rapid cooling and slow cooling characteristics, depend on the thermal conductivity of these dielectric layers, it is possible to optimize the recording and erasing characteristics by properly choosing the materials of the dielectric layers and by carefully controlling the thickness and composition of these layers.
Important design parameters which must be considered in developing and optimizing an erasable over-write optical recording medium are the erasability of the medium and the cyclability of the recording and erasing characteristics over many write/erase cycles.
With regard to the cyclability characteristics, studies have shown that there is a deterioration after a large number of write/erase cycles which results from thermal damage to the disc substrate or protective layer and which is manifested as an increase in noise. Further, studies have also shown that even in the absence of such thermal damage, a shift or physical distortion of the active layer along the direction of rotation of the disc may occur after many write/erase cycles as a result of thermally induced stress and distortion of the protective dielectric layers induced by the repeated heating and cooling cycles (see SPIE Optical Data Storage Topical Meeting, vol. 1078, p.27, Ohta et al.).
With regard to the erase characteristics, the melting point of non-crystalline films containing Te typically covers a wide temperature range of 400.degree. C. to 900.degree. C. As explained above, crystallization may be achieved by irradiating the active layer with laser light to increase its temperature, followed by a gradual cooling. The required temperature is generally within a range close to the crystallization temperature of the material, which is less that the melting point. When the crystallized film is irradiated with laser light having a higher power and is heated above the melting point, the film, upon rapid cool down, becomes substantially non-crystalline or amorphous, and an optically detectable mark is formed.
If the amorphous state is selected to represent the recorded condition, it is known that a more rapid cooling results in a more uniform amorphous state and results in a mark which produces a better and more stable signal. (See "Phase Change Disk Media Having Rapid Cooling Structure, Ohta et al., Jap. J. Appl. Phys. vol 28, 123 (1989)). These studies have shown that when the rate of cooling is too low, there arises a difference in the degree of non-crystallinity between the center of the mark and the periphery of the mark. During erasure, the mark is recrystallized. If the recorded mark is non-uniform in structure, the crystallization which occurs during subsequent erasure will be rendered non-uniform as well, resulting in a recording medium with less than optimum erasure characteristics.